State of art cathode material for rechargeable lithium batteries are transition metal oxides with layered crystal structure (r-3m) like LiCoO2, LiNiO2, LiCoxNi1−xO2, doped LiCoxNi1−xO2 (using dopands like Al, Mn, Mn1/2Ni1/2, Mg, Ti1/2Mg1/2), LiNi1/3Mn1/3Co1/3O2, Li(Ni1/2Mn1/2)1−xCoxO2, Li1+xM1−xO2 (M=Mn1−yNiy), LiCo1−x(Ni1/2Mn1/2)xO2 etc. Generally these materials are powders. These materials have different properties, but generally the composition in the outer bulk near to the surface of a particle is the same as the composition in the inner bulk, near to the center of the particle. Much work has been spent to optimize the composition and morphology for such cathode material. This approach can be summarized as the “uniform approach”. “Uniform” cathode materials have a composition, which is the same in the outer and inner bulk. A uniform composition can be simple (like Co in LiCoO2) or complex. Recent disclosures show that lithium transition metal oxides with complex metal composition have improved properties. Complex lithium transition metal oxides are described in various patents, without claim for completeness a few will be mentioned.
PCT—WO141238 A1 (Paul Scherrer Institute, Swizterland) with priority Dec. 3, 1999 describes complex materials basing on doped LiNi1/2Mn1/2O2, prepared from precipitated precursors.
U.S. Pat. No. 5,626,635 (Matsushita) describes LiNiO2 doped by Mn or Co.
U.S. Pat. No. 6,040,090 (Sanyo) describes materials having a certain X-ray diffraction feature. The patent covers a very wide selection of doped LiNiO2, where dopands are Mn, Co, Al etc. The patent claims also include complex materials, with composition LixMO2, where M is (Ni1/2Mn1/2)1−xCox.
EP1 189 296 A2 (Ilion) with priority Mar. 9, 2001 claims lithium rich materials Li1+xM1−xO2 with M=(Mn1−uNiu)1−yCoy with u≅0.5, x>0 and y<⅓.
US20030108793 and US20030027048 (3M) with later priority (Aug. 7, Apr. 27, 2001) claim an extremely wide range of solid state solutions within the quaternary system LiCoO2—LiNi1/2Mn1/2O2—Li[Li1/2Mn2/3]O2—LiNi1/2Mn1/2O2, but eventually focusing on materials which are solid state solutions of Li[Li1/2Mn2/3]O2 with LiNi1/2Mn1/2O2 or solid state solutions of LiCoO2— with LiNi1/2Mn1/2O2.
Excellent results of complex cathode materials were reported: Examples are materials like Li[LixM1−x]O2 with x≅0.05 and M=(Mn1/2Ni1/2)5/6Co1/6 (Paulsen&Ammundsen, 11th International Meeting on Lithium Batteries (IMLB 11), Cathodes II, Ilion/Pacific Lithium), LiNi1/3Mn1/3Co1/3O2 as demonstrated by Ohzuku (Makimura&Ohzuku, Proceedings of the 41st battery symposium on 2D20 and 2D21, Nagoya, Japan 2000 or N. Yabuuchi, T. Ohzuku, J. of Power sources 2003, (in print) ) or doped LiCoO2 demonstrated by Prof. Dahn's group (S. Jouanneau et all., J. Electrochem. Soc. 150, A1299, 2003).
Generally, the optimum of all uniform compositions is a compromise between many different requirements, like cost, processing and performance/properties. These requirements cannot be met simultaneously. This is because the requirements for the inner bulk, the outer bulk and the surface of the particle are different. Consequently, conventional materials with a uniform stoichiometry do not represent the optimum regarding cost, processing and properties, and as a general conclusion, materials with uniform composition are not fully optimized.
Previously, some work, usually called “surface coating”, has been spent to engineer cathode materials. The coating approach is for example described by Y. J. Kim et all., J. Electrochem. Soc. 149 A1337, J. Cho et all., J. Electrochem. Soc. 149 A127, J. Cho et all., J. Electrochem. Soc. 149 A288, Z. Chen et all., J. Electrochem. Soc. 149 Al 604, Z. Chen, J. Dahn, Electrochem. and solid-state letters, 5, A213 (2002), Z. Chen, J. Dahn, Electrochem. and solid-state letters, 6, A221 (2003), J. Cho et all., Electrochem. and solid-state letters, 2, 607 (1999), J. Cho and G. Kim, Electrochem. and solid-state letters, 2, 253 (1999), J. Cho et all., J. Electrochem. Soc. 148 A 1110 (2001), J. Cho et all., Electrochem. and solid-state letters, 3, 362, (2000), J. Cho et all., Electrochem. and solid-state letters, 4, A159, (2001), Z. Whang et all., J. Electrochem. Soc. 149, A466 (2002), J. Cho, Solid State Ionics, 160 (2003) 241-245.
In the surface coating literature, a small amount of a compound M2 is coated onto the surface of particles having the composition M1, usually followed by a heat treatment. Depending on the choice of M2 and M1, and depending on preparation conditions, two extreme cases exist: Case 1: A very thin layer around the particle has a significantly different stoichiometry than the bulk of the particle. Case 2: An extended region at the outside of the particle has a small change of stoichiometry.
To achieve performance optimized materials, it is however required, that both the change of stoichiometry is significant, as well as that the regions with different stoichiometry are extended.
A cathode material, usually with layered crystal structure (preferred LiCoO2) was coated by a gel, containing another metal, followed by a mild heat treatment. Metals of choice were Al, Sn, Zr, Mg, Mn, Co. In most cases, no solid state solution between LiCoO2 and the metal exist, so that the surface is partially or completely coated by a localized second phase, showing phase boundaries, and a steep change of stoichiometry across the boundary. The composition of inner bulk and outer bulk is the same. In some cases a solid state solution between the LixMO2 cathode and the coating metal exist. In this case a gradient of composition of the coated metal is achieved. However, generally, only small or very small amount of the metal were coated, not exceeding about 0.03 mol or less coating metal per 1 mol of LixMO2 cathode. As a result the composition M1, M2 in the inner bulk and the outer bulk differ significantly only for the coating metal composition, but not significantly (<10%) for the bulk transition metal.
Typical coated cathode particles are covered by a very thin, often incomplete surface of a metal oxide phase with a sharp phase boundary. It is discussed in the literature, that the coating prevents the contact of electrolyte with cathode, thus hindering unwanted side reactions. During charge-discharge, a cathode like LiCoO2 expands and contracts. It is speculated that a “hard” surface could mechanically prevent the exapansion-contraction, thus limiting the strain within the bulk, and causing better cycling stability. In this case a thin coating layer with relatively sharp interface between bulk and coating layer will experience significant local strain. A thin surface cannot accompany this strain, resulting in cracks, or loss of mechanical contact, thus making the coating layer less efficient. Additionally the coating material might be electrochemically inert, not contributing to the charge capacity of the cathode material. The surface layer of coated cathode powders is typically formed by a separated “wet” sol-gel step, followed by a mild heating. Sol gel technology is expensive, thus increasing the cost.
It also needs to be mentioned that the effectiveness of the coating is severe questioned. Recent publications by J. Dahns group (Z. Chen, J. Dahn, Electrochem. and solid-state letters, 6, A221 (2003) and Z. Chen, J. Dahn, Abs 329, 204th ECS Meeting, Orlando) show that not the coating, but rather the heat treatment of LiCoO2 is responsible for the observed performance improvement.
From a general point of view, coated cathode materials are not fully optimized. It is not sufficient to distinguish between different requirements for bulk and surface properties only, for a full optimization it is also required to optimize the cathode for different requirements for the properties in the inner bulk, the outer bulk and at the surface.
The actual invention is instead focusing on materials with significantly different transition metal stoichimetry in the inner bulk, outer bulk and surface. In cathode materials of this invention, the transition metal stoichimetry changes smoothly over an extended region of the particle. In cathode materials of this invention, the outer bulk expands or contracts in a similar way as the inner bulk. Therefore mechanical integrity is guaranteed. Furthermore, the modified layer is electrochemically active, therefore no capacity of the cathode is lost.
U.S. Pat. No. 6,555,269 (J. Cho et all., submitted by Samsung) describes LiCoO2 coated by metal selected from Al, Mg, Sn, Ca, Ti, Mn. The coating is made by sol-gel technique, followed by a mild heat treatment at 150-500° C. The maximum amount of metal per Co is 6%. The final cathode does not contain nickel. Furthermore, Ca and Sn (and probably Mg) do not form solid state solutions with LiCoO2. Manganese and titanium only form solid state solutions if fully lithiated to Li2TiO3 and Li2MnO3. This approach does not achieve a Mn—Ni—Co based material as described in the actual invention, where the manganese composition, as well as cobalt composition as well as nickel composition of inner bulk and outer bulk change significantly. Particularly, particles having a layered crystal structure and with smooth but significant spatial change of stoichiometry are not achieved.
U.S. Pat. No. 6,274,273 (J. Cho et all., submitted by Samsung) describes a an active material, being lithium manganese spinel, coated by cobalt, thus establishing a concentration gradient of cobalt. The cathode material is prepared coating the spinel by sol-gel, followed by a heat treatment below 850° C. The amount of Co (per Mn) is limited to below 5%. Only very small stoichiometric changes of the base manganese are achieved. FIG. 5a for example would lead to the conclusion, that the manganese stoichiometry only varies by about 1%, ranging from 0.963 at the surface to about 0.973 in the center of the particle. Neither the starting lithium manganese oxide nor the coating metal contain nickel. This approach does not deliver a Mn—Co—Ni material according the present invention, and having a significant change of manganese, nickel and cobalt stoichiometry.
J. Cho, Solid State Ionics, 160 (2003) 241-245 describes the coating of LiCoO2 by Li—Mn spinel particles. Basically a slurry, containing balhmilled microsized spinel is added to a slurry containing LiCoO2. The final material does not contain nickel. It is questionable, if a thick coating can be achieved at all by the described method. In the beginning, spinel might be coated onto the surface of LiCoO2. But, as soon as the surface is covered by spinel, further coating of spinel would require the attachment of spinel particles to the spinel covered surface of the LiCoO2 . This attachment, however, is not limited to the surface of LiCoO2 particles, it will also cause an uncontrolled agglomeration of spinel particles. As a result, either the surface is thin, or only partially coated, or significant agglomerates of spinel, without LiCoO2 core will be present. In fact, FIG. 1 shows different types of particles and indicates that a significant fraction of the particles are agglomerates of spinel without LiCoO2 core.
Some patents describe cathode materials with non-uniform composition. Improved electrochemical properties are for example reported for mixtures of LiNi1−xCoxO2 and lithium manganese spinel. These cathodes are however uniform (i.e. not non-uniform) in the sense of the actual invention because particles have the same composition in the outer and inner bulk.